The invention relates to a method of manufacturing a colour cathode-ray tube comprising a magnetic quadrupole post-focussing mask formed by a plate of a magnetisable material in which apertures are provided so as to be arranged according to rows, sections of said plate being successively magnetized by positioning parallel conductors per section on at least one side of the plate in such manner that each time two conductors through which an electric current flows in opposite directions are present between two successive rows of apertures, a magnetic quadrupole being formed along the circumference of each of the apertures.
The invention relates also to a device for carrying out such a method.
Such a method of manufacturing a colour cathode-ray tube comprising a magnetic quadrupole post-focusing mask is disclosed in Netherlands Patent Application No. 8002303. The object of magnetic post-focusing is to increase the transmission of the mask. In tubes without post-focusing a very large part, for example 80 to 85%, of the electrons is intercepted by the so-called shadow mask. When magnetic post-focusing is used the apertures in the mask may be made larger since as a result of the focusing in the mask apertures the electron spots on the screen are considerably smaller than the mask apertures so that sufficient space is available between the electron spots of the various electron beams. As a result of the presence of a magnetic quadrupole lens in each of the apertures of the mask, the electron beams upon passing a mask aperture are focused in one direction and are defocused in a direction at right angles thereto.
In the known method, magnetisation of the mask plate is done by means of a magnetizing device comprising two sets of parallel conductors which mutually are oriented substantially at right angles and are provided on either side of the plate. Two conductors through which an electric current flows in opposite directions of the respective systems always extend between two rows and two columns respectively, of mask apertures. The directions of flow in the two systems of conductors are chosen to be so that a magnetic quadrupole is formed along the circumference of each of the mask apertures. It is thus possible to permanently magnetize the mask plate throughout its surface in one operation. For practical reasons, however, it may be desired not to magnetize the entire mask plate in one operation but in sections. The energy required for the magnetization then need not be supplied at one time but may be supplied in successive steps per section. Moreover, smaller magnetisation coils may be used which need extend only over one section. As a result of this, the required size accuracy of the coils can more easily be met. The problem occurs, however, that when the known magnetisation device is used, a magnetisation which deviates from the desired magnetisation and which in the operating tube causes a visible disturbance in the displayed picture occurs in the boundary region between two individually magnetized sections. Investigations have proved that the deviating magnetisation formed in the said boundary region is caused by magnetic fields which are generated by the magnetisation device and extend beyond the sets of conductors. At the edges of the systems of conductors said so-called edge fields during magnetizing a given section disturbs the magnetisation of a previously magnetized adjoining section. In the known magnetization device the ends of the parallel conductors are interconnected in a zig-zag manner and bent at right angles to the plane through the conductors. By this measure, however, the interfering influence of the edge fields is removed only at the edges which are at right angles to the conductors. The disturbing influence of the edge fields at the edges which extend parallel to the conductors is not removed.